This invention relates to component color video signal measurement methods and apparatus, particularly to measurements of spectral and temporal distortion, and to measurements of the characteristics of time-division multiplexed component video signals.
In conventional color television systems the color characteristics of the video image, that is, the luminance, hue, and saturation, are oridinarily represented by three signal components. The components sometimes used are one luminance component and two distinct chrominance ("chroma") components. The two chroma components are typically synthesized from a weighted combination of red, green, and blue signal levels.
The conventional method of transmission of television signals in the United States is based upon a method of frequency-division multiplexing adopted by the National Television System Committee (NTSC) in 1953. In that system signals representing the luminance and chroma components are frequency-division multiplexed and transmitted simultaneously. The chroma signals are shifted in phase 90 degrees from one another and thereafter used to modulate the same subcarrier, which is suppressed prior to adding the luminance signal with the product resulting from the subcarrier modulation. In demodulation the subcarrier must be regenerated. Hence, the relative phase and amplitudes of the chroma signals and the subcarrier are important.
Modern technology has led to a trend toward the adoption of time-division multiplexed color television modulation. In these systems the luminance and chroma components are separated from one another in time and transmitted in sequence. For example, for each horizontal scanline a first time-compressed chroma component segment corresponding to that scan line is transmitted followed immediately by the transmission of a time-compressed segment of the second chroma component corresponding to the same scan line. The corresponding segment of the luminance component is transmitted immediately thereafter. The luminance segment also may be time-compressed. At the receiver, the first and second chroma segments, and possibly also the luminance segment, are time expanded, and the first and second chroma segments are delayed relative to the luminance segment in order to bring the three segments into time coincidence. Methods known as "time compressed color component" (TC3) and "multiplexed analogue component" (MAC) are variations of the foregoing scheme.
In time-division multiplexed systems there is no color subcarrier, so there is no need to measure the relative amplitude and phase of chroma components in the traditional sense. However, the relative timing between chroma component segments, and between the luminance and chroma component segments is important, as it affects the accuracy of the transition from one color to another in the received image. The bandwidth of the signal channels for all of the components is still important because it affects image definition, and the relative signal levels of the components are important because they affect the hue and saturation of the image produced.
A well known instrument for measuring the amplitude and phase characteristics of an NTSC frequency-division multiplexed signal is a vectorscope, for example, a Tektronix Model 520A NTSC vectorscope. As is commonly known in the art, such an instrument is essentially an oscilloscope having video signal decoding circuitry that provides to the horizontal and vertical deflection circuitry two signals representing the chroma components of the video signal. The oscilloscope trace is referenced to a polar coordinate system on the graticule, thereby providing a display indicative of the phase relative to subcarrier and amplitude of the color signals. Typically, positions are shown on a graticule over the display of the dots created by the trace representing selected saturated hues employed in the generation of a test pattern, ordinarily a color bar pattern. The trace transition between the dots is of little significance, since its shape is principally a function of the circuitry of the vectorscope itself. Thus, while such an instrument is valuable for measuring the relative phase and amplitude characteristics of chroma in an NTSC signal, it is not useful for measuring the spectral or transient characteristic of such a signal, nor is it useful for measuring the timing of a time-division multiplexed component color video signal.
Accordingly, it would be desirable to have a method and apparatus for measuring the spectral, timing, and amplitude characteristics of any type of component color video signal, and specifically for the measuring of a time-division multiplexed signal.